Retrieve errno() in a thread-safe way.

http://code.google.com/p/go/issues/detail?id=709#c3 shows how errno()
can be checked in a thread-safe way with a familiar idiom.

https://zeromq.jira.com/browse/LIBZMQ-338 shows the libzmq devs aren't
sure zmq_errno() should be used at all.  The libzmq source as far back
as v2.1.0 includes a one-line zmq_errno() implementation that only
returns errno, adding nothing to errno().
Merge pull request #54 from jtacoma/threadsafe-errno

Retrieve errno() in a thread-safe way.